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Hey Guys,
I've been working on a set amp kit for the last 3 months. I have built several other kits and this is by far the toughest for me. I made the mistake after opening the boxes and going thru all the parts accepting it, knowing this looks like someone just thru together what they had on the shelf. I only say this so we can troubleshoot the amp better.
As of last week I had not gotten any sound out of the amp. This week I started messing with the heaters on the 6550's. They are ac heated. Per spec it has the lines from the power transformer high side go into a .68 ohm resister to pin 2 (7) of other 6550. Low side feeds pin 7 (2) with a wire going to ground. With the manual that was sent, originally the power tranning had four wires for the heaters, now the tranny only comes with two wires for the heaters.
When I hook up the heaters per spec, I get no sound out of the speakers. The 6550's don't heat up at all. If I hook up the heaters in series, one channel will give music and the other channel little music. If I turn it up all all the channel with little music will start to oscillate.
If I hook up the heater wires, high side feeding both .68 resisters and run them pin2 of one 6550, pin 7 the other 6550. opposite of the low side but I don't hook up the ground. I get music out of both channels but only for about 3 minutes. Its starts oscillating. Volume will not stop it and I have to turn the power off.
Follow Ups:
Maybe I don't understand. You are getting sound out (albeit heavily distorted), even though your heaters are not working?
"When I hook up the heaters per spec, I get no sound out of the speakers. The 6550's don't heat up at all."
Are you saying if you hook up the heaters, as shown in the schematic, the heaters don't even light up?
But if you don't ground one side, leaving everything else the same, they do light up?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
yes, per schematic and pictures provided by manufacturer the 6550's will not heat up.
With the tubes out, and neither side of the filament winding grounded, measure the resistance of the filament winding to ground.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Did you try ohming out the 6550 filaments to see if they cooked?
I have no idea what number you should get, but they shouldn't be open (couldn't find the infinity key)
Kelly
You said the schematic shows four filament leads from the transformer. Yet yours only has two. I assume it also has a 5v rectifier winding that goes ONLY to the rectifier filament?JUST SPECULATION, but the schematic transformer may have had two 6.3v filament windings. The transformer they shipped has only one but at twice the current.
In any case the 6550 is a 6.3v filament. So if the transformer secondary is 6.3v, you would put both 6550s in parallel. If it's 12.6v then the 6550s would have their filaments wired in series.
CHECK THIS WITH ALL THE TUBES OUT! Running the tubes at 2x filament voltage will destroy the quickly cathodes if it doesn't just burn the filaments out right away.
You can get decent multimeters at any home center for less than $50. Harbor Freight also has them sometimes less than $10! This is a highly useful tool to have around the house, even beyond your tube projects. GET ONE TODAY!
Edits: 03/04/12 03/04/12 03/04/12
Yeah I have a multimeter,
Both the 12AX7's and 6SN7 have 6.3vac, The 6550's have 7.3vac before the R68 resisters.
With the 12ax7's and 6SN7 using center tap after the choke in the HV rail, the heaters for the 6550 have the low side grounded in the schematic. What would cause either 6550's to power? This is why I tried with no ground.
Very good questions guys, thanks. I am having troube uploading the schematic. I have apple computers so I will try with my other one later.
With 7.3 VAC before the 0.68 ohm resistor, what voltage to you measure across the 6550 heater with the tube plugged in?
*Grounding* the AC filament supply provides for less AC Hum introduced via the filament. Realize that the term *ground* no longer is a direct Earth connection as was neccessary at Radio Frequencies(RF) with crystal radios during the early days of electronics, but merely a vintage carry-over term for the return current path. There is no operational requirement for audio circuts to be Earth or Ground connected. If you think of so-called *ground* as the Direct Current return pathway (as the term actually means today), then circut operation becomes easier to understand. The filament circut powers up because the AC filament current has a completed circut regardless of the DC return connection (AC is Not shorted out, but because of that DC connection now has a component carried on the DC which nulls any out-of-phase AC Hum induced from the filament).
Edits: 03/05/12
here is the schematic
One way or another all the tubes need 6.3v at their filament terminals. The schematic shows separate filament windings for the driver tubes. I don't know why they think they need that but whatever. The centertaps of the driver tube windings are being biased to a DC voltage. This is a common practice for hum rejection. Power output tubes aren't as noise sensitive. In your case they simply tied one side of the power tube filament to ground. They should light with the ground connection or not. The only difference may be some hum with that ground missing.I do see something though. The 12AX7 should have pins 4&5 tied together. The schematic only shows pin 4. The 12xxx tubes are 12v/6v filaments. For 6.3v you tie pins 4&5 and connect the 6.3v between 4&5(shorted together) and pin 9. For 12.6v you connect the heater leads to pins 4 and 5 (not shorted together) and leave pin 9 unconnected. POSSIBLE EXCEPTION: If they are only using half the 12AX7, then you could just heat half of it as well using pins 4 and 9.
Now you said your actual transformer is missing one of the windings. Perhaps in that case both driver tubes should be connected to the same winding leaving that 7volt winding for the output tubes.
Audionote is a respected kit manufacture AFAIK, so can't you contact them for help?
Edits: 03/05/12 03/05/12 03/05/12 03/05/12
The 12ax7 heater's probably have Pins 4,5 traces in parallel. Needing only one connection for those two pins, then one > pin9.
So, I hooked up the heaters using the 7 vac per pictures from the building manual. I only got 3.3 acv with the tubes installed, 7.2 vac with them out. I measured the resistance across both tubes and they showed 1.1 ohms. I measured a second pair of 6550's from a VTL amp and they showed the same.
I went back and wired the heaters per schematic, high side to .68 ohms to pin 7 of one tube. I wired from pin 7 to other .68 ohm and left the resister to pin 2 of other 6550. I and ran a extra ground from the low side connecting pins 7 and 2. The ground was ran to the star ground that is on the power supply board. I had 8 ohm resisters on the opt terminals.
I decided to hook it back up to the speakers and run my ipod into it. It started up no problems but after a few minutes the left side died down and the right side died down a little bit. I decided to turn it off and go back in, I just start sweating so fast when I have it open and powered up. Thanks guys I think I am making progress.
Are the 6550s filaments in series or parallel? It sounds like you have them in series? Both tubes pin 7 and both pin 2 should be connected together. The pin 7 side to the resistor, the pin 2 side directly to the transformer.OR
You can criss cross the filament pins between the tubes as shown on the schematic. My guess is they did this for hum cancellation.
Take the 6550 tubes out. You should measure approx 6-7 volts between pins 2 & 7 on each socket. The measure again with only one tube plugged in, then with both 6550s in. Tell us what you get in each step.
Edits: 03/05/12 03/05/12
No distortion, one channel has sound the other does not at this time.
I have the heaters hooked up _(2)_(7), low side I have green wire running _ground___(2) ground.
Full screen with as much detail as possible so we can analyze the wiring.
empty sockets 7.42 vac
first 6550 6.09 vac
first 6550 5.4 vac second 6550 4.33 vac
off variac straight in wall first 6550 5.58 vac second 6550 4.65 vac.
just lost a 5u4gb in the process, thru in the backup.
other 6550
Got the new power tranny in this week, finished installing it last night. I first setup the 6550 heaters per picture in manual. Only 3.2 vac at the heaters of the 6550's. Went back and setup the heaters per schematic and I am now getting 6.25vac across both heaters with the tubes installed.
Here is the kicker, I hooked up the amp to a pair of speakers, only one side was working. However I let it play for over an hour to see if the original motor boating would happen. Nothing. Went to bed at 5 am this moring.
This afternoon I put a new 12ax7 on the side that wasn't working. I turned on the amp and the channel that was working last night started motor boating on me. I opened the amp back up and turned it on with 8 ohm resisters on the speaker term, I measured 6.25 vac on the 6550's.
I am done with it, I was going thru measuring voltages and smoke started escaping.
It looks to me like the tube in the back has it's 0.68ohm resistor being fed from the 0.68ohm resistor on the first tube. Both 0.68ohm resistors should be fed from the blue wire which I assume comes from the transformer. So move that red wire from the front tube socket to the terminal strip where the front 0.68ohm is tieed to.
That said it should still work they way you have it, one tube however will have less heater power.
Did you test the 0.68ohm resistors?
The soldering of the two blue wires and the terminal strip could use some cleanup. It looks like a short waiting to happen.
Standard pharmacy Isopropol alcohol will remove rosin flux. It takes a while though.I use Acetone. It's very fast and evaporates quickly. Just keep it away from plastic as it will eat it. However, not all states will allow sale of this product over the counter. Interestingly enough in California, the tree hugger state, you can buy Acetone by the gallons at any Home Depot or Lowes.
Edits: 03/06/12
It's hard to tell from the pictures.You said "I went back and wired the heaters per schematic, high side to .68 ohms to pin 7 of one tube. I wired from pin 7 to other .68 ohm and left the resister to pin 2 of other 6550."
This is a little confusing. forget about pin 2 and pin 7 being swapped. You can swap them or not.
The bottom line is, one filament of one 6550 should be connected to the filament winding through a .68 ohm resistor. The other filament of the other 6550 should also be connected to the filament winding through a .68 ohm resistor with nothing in series.
pay no attention to the type of tube in the drawing. That's the only tube symbol of a tube that showed the heater connections I could find.
If it doesn't work with one side grounded then you have a shorted (to ground) heater winding.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/05/12
I had it wired up this morning the way you are showing. I was getting 3.35 across the heaters. This was the reason I wanted to lift ground. The amp oscillated with this method.
You have a shorted heater (filament) winding.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I just shut down the amp and the channel that wasn't' working came on for a second. Would this also fall in line with a shorted winding?
I have no idea about that. That could be a separate problem.
The heater winding just powers the heaters in the tubes.
You said you got only 3.3vac hooked up the right way?
Try it without the resistors and measure what you get.
I really do think there is a problem with the heater winding of the power transformer.
BTW In a in-directly heated tube, like the 6550, they are called heaters, not filaments. We all say filaments even when we know better.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
took the resister off of one of the heaters, front 6550 7.35 vac second 6.33 vac.
well the oscellation returned with voltages of 5.34 on one tube and the other 5.6vac
I'm pretty much done with this thing. Thanks for all your help today. I will get hold of the manufacturer tomorrow and see what I can do about.
This isn't my first kit, probably my 9 or 10th kit and the first one to not work on me. Frustrating as heck. O well back to the dynaco 70 tonight.
Well Brian is sending me another power tranny in the next week. I will keep you all posted. Thanks again for your help.
Thanks for the heads up.
,
Have Fun and Enjoy the Music
"Still Working the Problem"
A good percentage of the time are caused by the layout and soldering of the internal components within the unit. An "under-the-hood" photo, along with the schematic may help narrow down some possible issues.
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